Stuffing boxes are used in various applications for sealing with a rod-like member which passes through the stuffing box and either reciprocates or rotates relative to the stuffing box. The stuffing box seals with the rod member to retain fluid pressure, which is generally below the stuffing box, thereby allowing the rod member to extend upward through a sealed chamber, which is typically fluid pressurized. A plurality of axially stacked packing rings or packing glands seal between the body of the stuffing box and the rod member. The packing rings are axially compressed or loaded by adjusting a gland member moveable relative to the stuffing box body, thereby exerting a compressive force on the packing rings to result in enhanced sealing. If the stuffing box leaks, the gland member is tightened to increase the loading on the packing rings, thereby resealing the packing rings with the rod member. Packing rings or packing glands of the stuffing box are thus distinguishable from conventional seals, which generally are not provided in a stacked arrangement, are not axially compressed by mechanical forces, and cannot be intermittently loaded to vary sealing effectiveness.
Stuffing boxes for oilfield operations typically are intended for sealing with a reciprocating or rotating rod string (or more particularly with a polished rod of a rod string) which in turn drives a downhole pump. In addition to oilfield operations, stuffing boxes are widely used to seal with rotating and/or reciprocating valve stems for various types of valves, and to seal with rods or pistons of pumps and other equipment which generate or handle pressurized fluid. Stuffing boxes for oilfield operations frequently present conditions which are, however, not typically encountered in other applications. Oilfield operations frequently are in remote locations, and accordingly stuffing boxes for sealing with sucker rods may be infrequently and irregularly serviced. Reciprocating sucker rods often become misaligned over a period of time with respect to the stuffing box, so that significant side loading forces are exerted on the packing rings by the reciprocating rod string. The downhole fluid sealed by the packing rings may contain a high level of contaminates which rapidly deteriorate the packing rings, including sand particles and other solids, hydrogen sulfide, or carbon dioxide. Oilfield pumps may also "run dry" during pumped-off conditions, or may produce fluids with high water content, and during these times the packing rings normally lubricated by the recovered downhole hydrocarbons are not lubricated. In applications where recovered hydrocarbons contain a high percent of steam or high temperature water which leaks past the stuffing box, a scale buildup on the sucker rod resulting from contaminants in the steam or water may quickly damage the packing rings.
The life of oilfield stuffing boxes and particularly the packing rings for oilfield stuffing boxes is therefore relatively low. A significant amount of fluid escapes from oilfield stuffing boxes with worn-out packing rings, thereby creating environmental concerns. A good deal of time and expense are associated with servicing and repairing oilfield stuffing boxes. The need has long existed for a more reliable oilfield stuffing box in which the packing rings can reliably seal with a sucker rod over an extended period of time.
U.S. Pat. Nos. 2,845,286, 3,168,320, 3,284,088 and 4,991,857 each disclose oilfield stuffing boxes for sealing with polished rods of a rod string. British Patent Nos. 307,207 and 446,869 each disclose packing rings having a generally rectangular cross-sectional configuration. U.S. Pat. No. 4,327,923 discloses a packing which utilizes rings having a triangular cross-sectional configuration to achieve a wedging action and force the surfaces of the rings into sealing engagement with both the exterior cylindrical surface of the stuffing box body and the shaft. U.S. Pat. No. 4,451,047 discloses a stuffing box having various rings formed of graphite, tetrafluoroethylene (TFE) polymers, and metal. Upper and lower surfaces of the rings are inclined and project radially inward and axially upward toward the packing retainer or gland cap.
U.S. Pat. No. 4,560,176 discloses an improved stuffing box with inverted packing rings which have tapered upper and lower surfaces projecting radially inward and downward, away from the gland cap. U.S. Pat. No. 4,480,842 discloses a stuffing box which is primarily designed for receiving a rotatable shaft or pipe. U.S. Pat. Nos.a 756,880, 882,103 and 1,184,212 each disclose packing rings for a stuffing box wherein the packing rings include low friction elements for sealing engagement with the rod passing through the stuffing box.
U.S. Pat. Nos. 2,081,040 and 4,268,045 disclose seals configured so that the seal material may expand into voids created by the seal configuration, thereby minimizing excessive generated heat and high seal wear. U.S. Pat. Nos. 3,393,917, 3,918,726 and 4,040,636 each disclose seals formed from a combination of rubber and plastic sealing elements.
None of the above patents provide a stuffing box which reliably seals with a rod member over an extended period of time, particularly when stuffing box packing rings are subjected to fluid which contains abrasives or chemical components deleterious to conventional sealing materials. The disadvantages of the prior art are overcome by the present invention, and an improved stuffing box and packing rings for a stuffing box are hereinafter provided for reliably providing a fluid tight seal around a rod member, and particularly around a polished rod used for oilfield recovery operations.